Synthesis and characterization of rhenium(VI) cis-dioxo complexes and examination of their redox chemistry

Reaction of rhenium(V)-oxo-halo-triflate complexes (HB(pz)(3)ReO(X)OTf (1, X = Cl, Br, I) with 1 equiv of pyridine N-oxide forms rare d(1) rhenium(VI) cis-dioxo compounds (HB(pz)(3))ReO2X (2, X = Cl, Br, I). This reaction likely occurs by initial formation of the d(0) rhenium(VII) dioxo cation (HB(pz)(3))ReO2X+ by oxygen atom transfer, followed by a rapid one electron reduction. The chloride derivative 2a has been characterized by an X-ray crystal structure. The d(1) dioxo compounds are fairly stable, disproportionating slowly to (HB(pz)(3))ReO3 and (HB(pz)(3))ReOX2. Electrochemical oxidations of (HB(pz)(3))ReO2X to Re(VII) cations are reversible and are at remarkably high potentials (E-1/2 = 0.93 V vs Cp2Fe+/0 in acetonitrile for 2a). When Me2SO is used as the oxidant instead of pyridine N-oxide, the Re(V) adducts [HB(pz)(3))ReO(X)(OSMe2)][OTf] (5) are formed by triflate displacement. These complexes reversibly lose SMe2 (for 5a, k = 3.3(4) x 10(-6) s(-1) at 297 K in CD2Cl2), as shown by isotope exchange experiments. The intermediate Re(VII) cations (HB(pz)(3))ReO2X+ oxidize Me2S much faster than Me2SO, indicating that they are highly electrophilic oxygen atom transfer reagents. Complexes 2, however, are relatively unreactive materials. Crystallographic data for 2a: C9H10BClN6O2Re; monoclinic, Cc; a = 14.716(3),b = 7.651(2), c = 13.232(3) Angstrom; beta = 1110.61(3)degrees; Z = 4.